A number of known control arrangements regulate the position or elevation of implements, such as plows, attached to or drawn by agricultural vehicles, such as tractors. Such control systems generally sense the position of a three-point hitch or other implement support structure and compare this position to a command value set by an operator using a command device. Based upon this comparison, such control systems generate control signals applied to valves which control the flow of hydraulic fluid to and from an actuator configured to vertically move the hitch, along with the implement mounted on it, to the desired elevation.
The hydraulic valves, which may include a raise valve and a lower valve or a three-position directional control valve, are typically solenoid-operated valves which include electrical coils. The coils operate the valves in response to electrical control signals generated by a control system. The control signals may include pulse-width-modulated (PWM) signals applied to drivers such that the rate of movement of the actuator is proportional to the duty cycle of the control signals. Typically, however, the control signal applied to each valve includes a threshold component designed to overcome inherent deadband in the valve and fluid flow forces within the valve, such as forces created by friction or springs, in order to open the valve and allow fluid to begin to flow through the valve. Thus, the control signal applied to the valve includes both a threshold component to open the valve and a component representing the desired drop or raise rate of the valve. The control signal is calculated by modifying a desired drop or raise signal by a gain value.
Known control systems, however, may experience problems which cause the implement to drop or raise in an undesirable manner to a position which is lower or higher than the desired position due to the gain value. This momentary jerk is disruptive to the operator and can unnecessarily shake the implement.
One such problem occurs because of valve hysteresis and the effect of implement weight on the movement rate. For example, a heavy implement may be commanded to elevate at a slow rate. To start the implement movement, a control signal including both a threshold component and a desired raise rate is applied to the raise valve. However, once the raise valve opens and the implement begins to move, the rate of elevation may be less than the desired elevation rate because the rate depends upon the weight of the implement. The system may overcompensate undesired rate gap by a gain value which results in a greater than expected elevation of the implement, causing a jerk felt by the vehicle operator.
Accordingly, a need exists for a implement lift system which adjusts the gain of the raise control signal to eliminate unwanted motion in the implement. Furthermore, there exists a need for a system which will adapt the gain of the control signal for future operations of the lift system to eliminate unwanted motion in the implement during the lifting and lowering operations.